This invention relates generally to surface acoustic wave devices, and, more particularly, to surface acoustic wave resonators for use in a variety of very high frequency applications in the communications and signal processing fields.
Surface acoustic wave (SAW) devices are a relatively new class of devices that utilize the propagation of ultrasonic acoustic waves in piezoelectric materials. In recent years, such devices have been advanced to a very practical stage of development, and they are now widely used in communications and signal processing systems. Ultrasonic waves provide the basis of a useful class of filters and resonators because such waves travel with negligible loss in suitable solids, at typical velocities of 10.sup.3 to 10.sup.4 meters per second. These velocities correspond to operating frequencies in the range of 30 to 800 megahertz (MHz) for practical surface acoustic wave devices.
When an acoustic wave is propagated along the surface of a piezoelectric material or substrate, a traveling electric field is also generated on the surface. This field also extends significantly above the surface, and can interact with appropriately constructed metal electrodes disposed on the surface. These electrodes typically take the form of an interdigital transducer, formed on the surface by photolithographic techniques. An interdigital transducer is a two-terminal device comprising a plurality of parallel metal fingers uniformly spaced on the surface of the piezoelectric substrate. The fingers are connected, usually in alternating fashion, to two terminal strips, giving the appearance of two combs with their teeth interleaved but not touching. When a voltage is applied across the terminals of an interdigital transducer, electric fields are generated within the substrate, and these generate corresponding stress patterns in accordance with the piezoelectric effect. If the voltage applied to the terminals is an alternating signal of suitable frequency, the value of which is dictated by the transducer finger spacings, elastic surface waves are launched in two opposite directions perpendicular to the transducer fingers.
Further background information on surface acoustic wave devices in general can be obtained from a variety of publications. For example, a book entitled "Surface Wave Filters, Design, Construction, and Use", edited by Herbert Mathews, and published by John Wiley and Sons, New York (1977), contains a good deal of useful background material on surface acoustic wave devices, as well as a comprehensive bibliography on the subject.
In a typical surface acoustic wave filter device, another interdigital transducer is used to receive the transmitted acoustic waves and to convert them back into an electrical signal. A resonator, on the other hand, operates by reflecting the propagated wave back on itself many times. An acoustic wave resonator comprises an interdigital transducer of the same general type just described, and two reflection gratings positioned one on each side of the transducer to form a resonant cavity in which the waves propagated from the transducer are repeatedly reflected back on themselves. The reflection gratings can be designed to have a reflection coefficient of almost unity at a selected frequency, and if the distance between the inside edges of the two gratings is an integral multiple of half-wavelengths, waves reflected back and forth between the two gratings will have the same phase and will add coherently. The cavity then resonates at the frequency of maximum reflection, and a standing wave is generated within the cavity.
The trend in the design of surface acoustic wave resonators has been toward higher and higher frequencies of operation, this trend being aggravated by an increasingly crowded communications frequency spectrum, and by the increased use of spread spectrum techniques in communications. Such higher operating frequencies dictate the use of correspondingly smaller grating element spacings and interdigital transducer finger spacings. As the resolution limits of photolithographic processes are approached, it becomes less practical to manufacture such devices capable of operation at high frequencies. It would, therefore, be desirable to be able to construct a resonator in such a manner that it would be capable of operation at frequencies higher than the fundamental frequency. The present invention is directed to this end.